Electronic components and in particular computer processors generate heat in operation, which can lead to overheating and consequent damage to the component and other parts of the system. It is therefore desirable to cool the component to transfer the heat away from the component and maintain the component temperature below the maximum operating temperature that is specified for correct and reliable operation of the component.
This issue especially concerns data processing or computer server centres, where a substantial number of computer processors are co-located and intended for reliable, continuous operation over a long time period. These centres may typically contain many server units, occupying multiple equipment racks and filling one or more rooms. Each server unit contains one or more server board. A single server board can dissipate many hundreds of watts of electrical power as heat. In existing systems, the energy required to transfer heat continuously so as to maintain correct operation can be of the same order of magnitude as the energy required to operate the servers.
The heat generated can be transferred to a final heat sink external to the building in which the processors are located, for example to the atmospheric air surrounding the building. Current implementations typically rely on air as the transfer medium at one or more stages between the processors and the final heat sink.
However, it is difficult to use air as a transfer medium for such a large quantity of heat, without imposing significant limitations on the building infrastructure. This is because the rate at which heat can be transferred increases with: increasing temperature difference (ΔT) between the heat source (such as the server boards, in particular the computer processors) and final heat sink; and decreasing thermal resistance of the path or paths thermally connecting the heat source and final heat sink.
Some known technologies for dealing with this difficulty are designed to control the environmental conditions of the location at which the processors are housed. Air handling techniques are currently often used, for example: vapour-compression refrigeration (“air conditioning”) of the air that reduces the local air temperature to increase the local temperature difference; and air pressurization (by the use of fans) to increase the air flow rate and thereby reduce the thermal resistance. Further heat exchange stages may be used to transfer heat extracted from the local air to a final heat sink, such as atmospheric air.
However, these approaches can be inefficient, as the use of air conditioning can require substantial amounts of electrical power to operate. These approaches can also make the location unpleasant for people, due to the local temperature and noise.
Furthermore, air flow rates and air temperatures may have to be limited, for example maintaining temperature above the “dew point” to prevent water vapour condensing out of air that may damage sensitive electronic components. For these reasons, servers are currently commonly distributed sparsely in order to reduce the heat density and improve local air flow, thereby reducing the thermal resistance.
Cooling the electronic components using a liquid that is brought into contact with the electronic components can be used to increase server density, reduce cooling costs or both.
An existing technique for cooling electronic components using a liquid is described in US-2007/109742 and GB-A-2432460. A computer processor board is housed inside an airtight container. A coolant liquid, preferably oil, is pumped through the container. The processor board is located at the bottom of the container and an evaporator coil is positioned at the top of the container, such that convection currents are produced in the coolant liquid. The coolant liquid is heated by the processor board and resultant vapour flows into a condenser. The container is positioned such that the circuit board inside lies in a horizontal plane to allow convection of heat from the components.
Using a condenser to provide refrigeration increases the complexity and cost of the system, and introduces further limitations on the system implementation.
WO-2006/133429 and US-2007/034360 describe an alternative known approach for cooling electronic components. The electronic component is sealed inside a container filled with a liquid and a thermally conductive plate is provided as part of the container in contact with the liquid. The thermally conductive plate conducts heat from the liquid to the outside of the container. Although this is designed for independent operation, the thermally conductive plate can be coupled to a further heat exchanger for additional cooling of the electronic component.
This alternative arrangement reduces the complexity of the system in comparison with approaches requiring pumped fluids inside the container. However, this does not significantly address the difficulty in reducing the thermal resistance between the heat source and the final heat sink. Even if the temperature difference is increased, the total thermal resistance will still be significant.